PROJECT SUMMARY/ ABSTRACT Metastasis remains the primary cause of breast cancer-related mortality yet there are no pharmacological inhibitors that target it. Metastatic behavior in breast cancer cells is currently thought to be induced by signals from the tumor microenvironment. Recent advances have shown that bone-marrow derived mesenchymal stem cells (MSCs), that home in to the sites of breast cancer growth in large numbers, promote metastasis in breast cancer cells. The mechanism underlying this was previously shown to be secretion of the chemokine CCL5 by MSCs. However, CCL5 secretion is not increased when some breast cancer cell-lines interact with MSCs even though they become more metastatic, suggesting that alternative mechanisms exist. To identify other mechanisms of metastasis promotion by MSCs, we performed gene expression analyses of MSCs and breast cancer cells after they had interacted in vitro. Network analysis identified type I interferons at the node of the most significant network of genes upregulated by those interactions. We hypothesize that type I interferon signaling is also activated when MSCs interact with breast cancer cells in vivo and this pathway is required for the MSC promotion of metastasis. Our hypothesis is supported by preliminary studies that show that interferon associated genes (IFGs) are highly overexpressed in the stroma of invasive breast cancers compared to normal stroma and outperform CCL5 as discriminators of invasive cancer stroma. Meta-analysis of whole-tumor derived gene expression datasets revealed that 11 IFGs, but not CCL5, are significantly associated with poor survival. Further, knockdown of a key transcription factor of the interferon pathway inhibits MSC-induced breast cancer cell migration in vitro. The major goals of this proposal are to identify the component of the interferon pathway that is the best biomarker of invasive breast cancer stroma, determine if interferon signaling is necessary for MSC-induced metastasis and to identify small-molecule inhibitors of the pathway that could be used as metastasis inhibitors. Biomarkers will be identified by RT-PCR and immunohistochemistry of tumor specimens derived from patients with early stage breast cancer who either did or did not ultimately develop metastasis. The necessity of interferon signaling for MSC-induced metastasis will be determined by knockdown of key components of interferon signaling and studying effects first on MSC- induced breast cancer migration in vitro, then MSC-promoted metastasis in vivo in a NOD-SCID xenograft model. Small-molecule inhibitors will be identified by assembling a collection of novel small molecules that inhibit regulators of the interferon pathway, determining if any of them inhibits the expression of the key components identified in previous aims then measuring effects on MSC-induced breast cancer cell migration in vitro. In the future, the best small molecule will undergo optimization and pharmacokinetic characterization prior to in vivo testing. The proposed studies are ideally suited for my goal of transitioning into an academic physician-scientist specialized in small-molecule inhibitors of the breast cancer microenvironment.